1. Field of the Invention
The present invention relates to an electric induction heater. More particularly, the invention relates to an induction heating coil for heating small-diameter, bar-shaped workpieces prior to their being forged.
2. Description of the Related Art
A basic challenge in designing a furnace to heat workpieces intended for use in a forging process is providing uniform heating of the workpiece as well as minimizing the time during which the heated workpiece is removed from the effective heating portions of the coil prior to being transported to the forging station.
Conventional electric induction furnaces used for heating small-diameter (less than one inch) bars intended for forging have various configurations. These configurations meet this challenge in different ways, none of which has proven as successful as desired.
According to one design, the workpieces are lined up end to end and are conveyed along their longitudinal axes through a circular cross-section induction coil that produces magnetic field lines substantially parallel to the workpiece axes. In addition to the general problem of cooling between heating and forging, non-uniform cooling along the length of the bar-shaped workpiece is a particular problem in this furnace configuration. The differential between the time the forward end of the workpiece exits the induction coil and the time the rearward end exits the induction coil creates a substantial temperature gradient along the length of the workpiece.
In another configuration, the workpieces are arranged with their axes parallel to each other and are conveyed through a wide oval or rectangular cross-section induction coil in a direction orthogonal to their longitudinal axes. An example of a conventional wide oval coil is described in U.S. Pat. No. 3,424,886, the disclosure of which is incorporated herein by reference to the extent necessary to achieve a thorough understanding of the background of the invention. As a practical matter, bar-shaped workpieces can be conveyed through such a coil in either of two directions, with the magnetic field lines of the coil parallel or orthogonal to the workpiece axes. In either case, coil end effects adversely effect the heating process.
In the case where the magnetic field lines are parallel to the workpiece axes, the workpiece is inserted along its longitudinal axis into one side of the opening of the coil, is rolled to the opposite side of the coil in a direction substantially parallel to the coil windings, and is then removed from the coil along its longitudinal axis. As is well known in the induction heating art, the magnetic field strength is lower at the ends of an induction coil than in the middle. Consequently, the axial ends of the workpiece, which are subject to the magnetic field of only the last one or two turns at each end of the coil, have a lower temperature than the middle portion of the workpiece. This temperature gradient is undesirable during the subsequent forging process.
Substantially uniform heating of a bar-shaped workpiece in a wide oval or rectangular cross-section coil can be achieved by conveying the workpiece through the coil with its longitudinal axis orthogonal to the magnetic field lines, that is, substantially parallel to the greater-length, horizontal segments of the coil windings. In this instance the workpiece is rolled from one open end of the coil to the other along the coil axis. Typically, however, this arrangement leaves the workpiece in the "dead" end of the coil for a period of time that is often substantially greater than the time it takes to transport the heated workpiece from the coil to the forging station, undesirably prolonging the cooling period.
The present invention is intended to provide an electric induction heater for heating bar-shaped workpieces that eliminates coil end effect problems.
The present invention also is intended to provide a wide oval or rectangular cross-section induction coil that heats bar-shaped workpieces uniformly and enables removal of the workpieces from the coil while they are still subject to a substantial magnetic field.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the apparatus particularly pointed out in the appended claims.